Rheological Considerations for Dual-Extrusion Melt Processing of Dissimilar Polymers in Composite Structures

Gel spinning is the current industrial method of choice for combining ultra-high molecular weight (UHMW) polymer resins with a substrate support polymer resin to produce composite filaments with a porous structure and high surface area per unit volume (specific area). Gel spinning is typically used to overcome a wide gap between the maximum processing temperature of the UHMW resin and the minimum processing temperature of the substrate resin and to avoid the high melt viscosity of the UHMW resin, but requires the costly recovery of toxic solvents. The UHMW resin is used because it forms a stable gel phase in the presence of water; a lower molecular weight resin (LMW) simply dissolves. A dual-extrusion process, which minimizes residence time with mismatched temperatures, was used to render a melt-based scheme practical. Dual-extrusion involves the separate plastication of materials prior to combination in a low residence time mixing head to form a desired composite. In this work, the UHMW and LMW resins were both poly(ethylene oxide) (PEO), and the substrate was polyarylsulfone (PAS). The initial focus of this dissertation is to investigate the rheology of PEO when subjected to temperatures beyond which it is known to degrade. Literature indicated PEO undergoes non-oxidative thermal degradation above 200°C and PAS is processed up to 350°C. Dynamic oscillatory shear rheometry was used to study 0, 25, 40, 50, 60, and 75wt% UHMW PEO in LMW PEO to take advantage of the sensitivity of viscosity to changes in molecular weight and material configuration, indicating degradation. Samples were exposed to 220, 230, 240, 250, 275, and 300°C temperatures for 5 minutes to explore conditions that could result in sample degradation. The viscosity decreased less with increasing UHMW PEO content for samples exposed to the same temperature and the viscosity decreased more with increasing exposure temperature for samples with the same UHMW PEO content. Parameters were regressed from observed data to predict the change in molecular weight via empiricisms relating the viscosity to molecular weight, shear rate, temperature, and time. This regression yielded a single master curve describing the behavior of PEO across all conditions, stable and degrading. The purpose of the second part of this work is to investigate the utility of the correlation developed with PEO in the first part with respect to characterizing an additional polymer resin, PAS, predicting the processing conditions for combining PEO and PAS in the dual-extrusion process, predicting the degradation of PEO in the dual-extrusion process, and characterizing the structure of the resulting composites with comparison to expectations from literature. The overall goal of eliminating the need for a toxic solvent in phase inversion gel spinning by changing to a melt process with dual-extrusion leaves theory and enters practice in this part. The correlation developed for PEO in the first part was used to regress parameters for PAS, extending the use case to an additional class of polymer resin. The regressions for both PEO and PAS were used to select processing conditions for operating the dual-extrusion process to yield composite filaments. Samples were produced with a range of compositions and prepared for microscopy as is, after etching with water, or after rinsing with water to remove extractables. Extractable content was characterized by the change in dry mass before and after rinsing samples using optical and scanning electron microscopy techniques. The observed excess extractables content of rinsed samples agreed with prediction from the regression for PEO and microscopy indicated qualitatively similar structure to similar gel spun materials in literature.